Word Origin & History
suffix forming almost all Mod.Eng. plural nouns, was gradually extended in M.E. from O.E. -as, the nom. plural and acc. plural ending of certain "strong" masc. nouns (cf. dæg "day," nom./acc. pl. dagas "days"). The commonest Gmc. declension, traceable back to the original PIE inflection system,
it is also the source of the Du. -s plurals and (by rhotacism) Scand. -r plurals (e.g. Swed. dagar). Much more uniform today than originally; O.E. also had a numerous category of "weak" nouns that formed their plurals in -an, and other strong nouns that formed plurals with -u. Quirk and Wrenn, in their O.E. grammar, estimate that 45 percent of the nouns a student will encounter will be masc., nearly four-fifths of them with gen. sing. -es and nom./acc. pl. in -as. Less than half, but still the largest chunk. The triumphs of -'s possessives and -s plurals represent common patterns in language: using only a handful of suffixes to do many jobs (cf. -ing), and the most common variant squeezing out the competition. To further muddy the waters, it's been extended in slang since 1936 to singulars (e.g. ducks, sweets, babes) as an affectionate or dim. suffix. O.E. single-syllable collectives (sheep, folk) as well as weights, measures, and units of time did not use -s. The use of it in these cases began in M.E., but the older custom is preserved in many traditional dialects (ten pound of butter; more than seven year ago).
third pers. sing. pres. indic. suffix of verbs, it represents O.E. -es, -as, which began to replace -eð in Northumbrian 10c., and gradually spread south until by Shakespeare it had emerged from colloquialism and -eth began to be limited to more dignified speeches.